US4034179A - Method of multiple electrode gas shielded arc welding - Google Patents

Method of multiple electrode gas shielded arc welding Download PDF

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Publication number
US4034179A
US4034179A US05/633,020 US63302075A US4034179A US 4034179 A US4034179 A US 4034179A US 63302075 A US63302075 A US 63302075A US 4034179 A US4034179 A US 4034179A
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Prior art keywords
gas
electrode
shielding gas
leading
welding
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US05/633,020
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English (en)
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Fusao Koshiga
Jinkichi Tanaka
Itaru Watanabe
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JFE Engineering Corp
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Nippon Kokan Ltd
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    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
    • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
    • B23K9/00—Arc welding or cutting
    • B23K9/16—Arc welding or cutting making use of shielding gas
    • B23K9/173—Arc welding or cutting making use of shielding gas and of a consumable electrode
    • B23K9/1735—Arc welding or cutting making use of shielding gas and of a consumable electrode making use of several electrodes

Definitions

  • the present invention relates to improvements in the automatic arc welding processes and more particularly to a method of multiple electrode gas shielded arc welding.
  • the submerged arc welding process and the gas shielded arc welding process are the popular welding processes among the known automatic arc welding processes.
  • a disadvantage of the submerged arc welding process in which a flux is used to serve the purposes of deoxidization and addition of alloying elements is that the removal of the flux is troublesome, the flux is expensive and the equipment tends to become bulky and expensive since it requires additional devices such as a flux feeding device.
  • the work is arc welded by using a pure gas such as argon or carbon dioxide gas or a mixed gas such as a mixture of argon with carbon dioxide gas or oxygen to exclude the entry of the air, particularly oxygen and nitrogen and therefore this welding process has the disadvantage of slow welding speed though it is free from the above-mentioned deficiencies of the submerged arc welding process.
  • the welding processes have been used frequently in which two or more electrodes are arranged in a straight row along the welding seam line of a piece or pieces to be welded for continuously accomplishing the desired multi-layer welding with the multiple electrodes. While this type of process has the effect of improving the welding efficiency to some extent, there is a serious disadvantage that there is non-uniformity of the properties among the different positions of the weld metal, particularly the impact properties at these positions differ considerably from one another.
  • the chemical composition of the shielding gas for the preceeding pass is different from that of the shielding gas for the following pass in such a manner that there is a relative difference in the volume ratio of the active gas in the shielding gas between the preceeding pass and the following pass.
  • the chemical composition of the shielding gas is controlled in such a manner that the volume ratio of the active gas in the shielding gas for the leading electrode or electrode group is higher than that in the shielding gas for the trailing electrode or electrode group
  • the chemical composition of the shielding gas is controlled in such a manner that the volume ratio of the active gas in the shielding gas for the leading electrode or electrode group becomes lower than that in the shielding gas for the trailing electrode or electrode group and in this way the weld metal is deposited in a plurality of layers.
  • FIGS. 1(a) and 1(b) are schematic diagrams showing respectively the manner in which the weld metal deposited by each pass is diluted with the base metal.
  • FIGS. 2(a) and 2(b) are graphs showing the results of the tests conducted for the purpose of calculating the cooling speeds and the equivalent input heats in the multiple electrode gas shielded arc welding process.
  • FIG. 3 is a schematic diagram showing an exemplary form of the groove shape used in working the present invention.
  • FIG. 4 is a graph showing the correlation between the welding current and the depth of penetration by a single welding electrode.
  • FIG. 5 is a graph also showing the correlation between the welding current and the amount of deposition of the electrode wire.
  • FIG. 6 is a graph showing the effect of the composition of the shielding gas upon the arcing phenomenon which is determined by the arc voltage and the arc current.
  • FIG. 7 is a schematic diagram showing the location of the notch formed in the Charpy test pieces used in the examples of the invention.
  • FIG. 8 is a graph showing an exemplary form of the transition tendency of the Charpy impact value when the volume ratio of the active gas in the shielding gas for the leading electrode is fixed and the volume ratio of the active gas in the shielding gas for the trailing electrode is varied.
  • FIG. 9 is an explanatory diagram showing the locations of the notch formed in the Charpy test specimens used in another example of the present invention.
  • the area of the groove face that would be melted by the first pass would be greater than in the case of the second pass with a resultant difference in the degree of dilution of the weld metal with the base metal between the two passes, namely, the rate of dilution of the weld bead layer made by the first pass would be greater than that of the weld bead layer made by the second pass.
  • the chemical composition of the respective layers in the resulting weld metal differs from one another according to their positions in the direction of the groove depth.
  • the base metal and the welding material contain alloying elements such as C, Mn, Si, Cr, Al, Ti, Zr, B, etc., which are easily combined with oxygen and the contents of these elements are high in high quality steels such as high tension steels and low temperature steels.
  • the problem of promoted deoxidation reaction by the effect of preheating is as follows. That is, while the above-mentioned deoxidation reaction takes place when the globule from the forward end of the electrode wire melted by the arc passes through the arc, before the globule solidifies in the molten pool and partly after the solidification, the deoxidation reaction of the weld metal tends to be promoted considerably since the cooling rate of the weld metal is slowed down as the welding input heat is increased and higher the preheating and interlayer temperatures become thus maintaining the weld metal at an elevated temperature for a longer period of time.
  • the base metal contains considerable amounts of alloying elements and the multi-layer welding is accomplished by the multiple electrode automatic arc welding process, due to the combined action of the above-mentioned two effects, namely, the preheating effect and the effect due to the difference of dilution rate, there results a still wider variation in the chemical composition of the weld metal of the bead layers deposited by the respective passes and this in turn results in a wider variation in the properties of the weld metal at different positions.
  • the dilution rate showing the extent in which a certain alloying element of the welding electrode wire in the weld metal is diluted by the base metal differs depending on the position of the bead layers in the case of a multi-layer welding and the dilution rate of the second and subsequent layers becomes increasingly smaller as compared with that of the first layer, namely, the alloy content becomes closer to that in the weld metal made by the electrode wire alone.
  • FIGS. 1(a) and 1(b) showing by way of example the case of welding two steel plates by depositing the weld metal with two passes. Referring to FIG.
  • FIG. 2 shows an example of the test results for calculating the equivalent welding heat inputs for the second or subsequent pass when welding a steel plate of 25 mm thick by the two-electrode tandem sequence gas shielded arc welding process from both sides of the plate, one run for each welding electrode.
  • the welding was effected by changing the distance between the leading and trailing electrodes to vary the interpass temperature between the leading and trailing electrodes.
  • the measurement made by inserting a thermocouple in the weld metal of the following bead indicated that the time required for cooling from 800° C. down to 500° C. was on the order of 25 seconds as shown in FIG. 2 (b) and this is equivalent to the welding heat input of 40K.J/cm when compared with the cooling time in the case of the single electrode welding as shown in FIG. 2(a).
  • the method of gas shielded arc welding according to the invention may be applied to any of processes employing electrodes consisting of wires having a large diameter or wires of a small diameter, provided the method employs a plurality of electrodes. Also the number of electrodes is not limited to two. Further, the types of shielding gas used with the present invention are not limited and various shielding gases may be used, such as, a mixed gas consisting of argon mixed with oxygen or a mixed gas consisting of argon mixed with carbon deoxide gas. Helium may also be used as the shielding gas in place of argon.
  • the chemical composition of the shielding gas used for the leading electrode or electrode group is selected to differ from that of the shielding gas for the trailing electrode or electrode group to vary particularly the volume ratio of the active gas contained in the shielding gas and in this way the required multi-layer welding is accomplished.
  • Which of the shielding gases for the leading and trailing electrodes or electrode groups has a higher active gas volume ratio than the other is suitably determined by the fact that which of the base metal and the welding material contains greater amounts of those elements which are easily combined with oxygen, namely, depending on whether the welding material is a low alloy material of high alloy material in relation to the base metal.
  • the volume ratio of the active gas in the shielding gas for the leading electrode or electrode group is selected high and that of the active gas in the shielding gas for the trailing electrode or electrode group is made low.
  • the amount of oxygen fed to each of the preceeding and following passes is accurately controlled so that the deoxidation reaction is promoted in the preceeding pass having a higher rate of dilution than the following pass, while the deoxidation reaction is checked in the following pass which is preheated by the heat energy provided by the leading electrode, and in this way the composition of the weld metal made by the preceeding pass may be brought close to that of the weld metal made by the following pass.
  • the amount of oxygen fed is controlled by making low the volume ratio of the active gas in the shielding gas for the leading electrode or electrode group and making high the volume ratio of the active gas in the shielding gas for the training electrode or electrode group, and in this way the weld metals made by the respective passes may be made of the compositions similar to one another.
  • the shielding gas composition namely, the definite volume ratios of the active gas in the shielding gas
  • they may be suitably selected in accordance with the electrode wire diameter, the composition of the base metal and the weld metal, etc.
  • the composition of the shielding gas for the leading and trailing electrodes respectively, the control of deoxidation reaction that suits the amounts of deoxidizing elements can be easily accomplished and in this way the weld metal can be accurately homogenized.
  • the multi-layer welding is also accomplished by adjusting the composition of the shielding gas for the preceeding pass relative to that of the shielding gas for the following pass to considerably improve the welding properties and thereby to produce high quality welded joint metals.
  • FIG. 3 shows the groove shape employed in the manufacture of steel tubes (wall thickness is 25 mm) by the method of multiple electrode tandem sequence gas shielded arc welding according to a preferred embodiment of the invention in which a single run each of the respective electrodes is applied on each side of the material
  • FIG. 4 shows the relationship between the welding current and the penetration depth in the single electrode gas shielded welding
  • FIG. 5 shows the relationship between the welding current and the amount of deposited electrode wire in the single electrode gas shielded welding.
  • the depth of weld penetration is more than 4.5 to 5.0 mm at the least and thus it will be seen from FIG. 4 that the welding current value for the leading electrode must be 800 amperes in the tandem sequence gas shielded welding.
  • the welding current value is selected 800 amperes
  • the amount of deposited electrode wire is on the order of 195 gr/min. Therefore, if the welding speed is selected 600 mm/min, then the groove of the shape shown in FIG. 3 will be filled with the weld metal from the electrode wire up to a level which is about 0.5 mm below the surface. Consequently, to fill the remaining slight space in the groove and obtain the proper reinforcement weld height (usual height is over 3.0 mm) and the proper weld bead width (usual width is 20 to 22 mm), it is essential to deposit a weld metal of about 150 to 160 gr/min by the following pass and use the same welding speed of 600 mm/min as the preceeding pass.
  • the proper welding current value for the following pass should be 700 amperes as will be apparent from FIG. 5. It is a well known fact that in the gas shielded arc welding it is generally desirable to effect the welding in the spray arc region, and if the welding is accomplished in the globule arc region or the short-circuiting region, it is impossible to obtain the proper peneration depth and moreover a considerable amount of spatter is caused thus deteriorating the welding properties considerably.
  • FIG. 6 is a graphic representation showing the manner in which the arcing phenomenon which is dependent on the arc voltage and welding current is varied depending on the volume ratio of the active gas in a shielding gas, and this will be explained in greater detail by taking the case of the above-mentioned embodiment.
  • the amount of active gas required for producing a spray arc condition is 15% if carbon deoxide gas (CO 2 ) is introduced into argon, while 5% of CO 2 may be properly introduced into argon to provide the similar spray arc condition by supplying the welding current of 700 amperes to the trailing electrode.
  • CO 2 carbon deoxide gas
  • the preceeding pass is effected by using as the shielding gas a mixture of argon with 15% of CO 2 and the following pass is effected by using as the shielding gas a mixture of argon with 5% CO 2 , it is possible to prevent the occurrence of spatter.
  • Table 1 shows the test results on the properties of the product obtained by using the same volume ratio of carbon deoxide gas in the shielding gas for both the leading and trailing electrodes (the conventional method) and the product obtained by varying the volume ratio of carbon deoxide gas (the method of this invention).
  • N1 represents the case wherein the notch was formed in the weld metal largely made by the first pass and N2 represents the case wherein the notch was formed in the weld metal largely made by the second pass.
  • the accurately homogenized weld metal was obtained by increasing the volume ratio of the active gas in the shielding gas for the leading electrode and decreasing the volume ratio of the active gas in the shielding gas for the trailing electrode.
  • the multiple electrode automatic gas shielded arc welding of the plate was accomplished using the same welding conditions as stated in (1), (2) and (3) of the first Example and increasing the CO 2 volume ratio of the shielding gas for the trailing electrode over that of the shielding gas for the leading electrode.
  • Table 2 shows the similar test results as the first Example.
  • the CO 2 volume ratio of the shielding gas for the trailing electrode may be increased in relation to that of the shielding gas for the leading electrode to bring the chemical compositions of the weld metals made by the respective passes close to each other and thereby to ensure the homogeneous properties throughout the entire weld metal.
  • Example 1 Using the electrode wires and base metal of the same characters as the Example 1, the same welding conditions as the Example 1 regarding the wire diameter and welding current, voltage and speed and a mixed gas of Ar+ O 2 as the shielding gas, the multiple electrode automatic gas shielded welding of the plate was accomplished.
  • Table 3 shows the similar test results as the Example 1.
  • the method of this invention is also effective in homogenizing the properties of the respective layers in the weld metal even when a mixture of Ar+ O 2 is used as the shielding gas.
  • the method of this invention is also highly effective when it is applied to the one-side welding involving the use of small diameter electrode wires.
  • Example 5 shows the similar test results as the Example 1.
  • the method of this invention is capable of performing with excellent results the one-side multi-layer welding using small diameter electrode wires and either of the mixture gases, and moreover the control of the shielding gas composition for the leading and trailing electrodes provided according to the method of the present invention includes the cases where the active gas is reduced to zero.
  • the method of this invention is capable of producing excellent products when the shielding gases having different compositions are used for the leading and trailing electrodes and the wires having different chemical compositions are used as the leading and trailing electrodes.
  • the multiple electrode gas shielded arc welding is accomplished by controlling the composition of the shielding gas for the leading or trailing electrode or electrode group relative to that of the shielding gas for the other and in this way the chemical composition of the weld metal made by each pass is accurately controlled to properly homogenize the properties of the respective layers in the entire weld metal and solve the problem of the occurrence of spatter, thereby ensuring an improved welding efficiency and operating properties and producing weld zones of excellent properties.
US05/633,020 1974-11-26 1975-11-17 Method of multiple electrode gas shielded arc welding Expired - Lifetime US4034179A (en)

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JP49135181A JPS5236941B2 (de) 1974-11-26 1974-11-26
JA49-135181 1974-11-26

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JP (1) JPS5236941B2 (de)
CA (1) CA1040718A (de)
DE (1) DE2552495C2 (de)
ES (1) ES442954A1 (de)
IT (1) IT1049705B (de)

Cited By (9)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4100389A (en) * 1975-12-24 1978-07-11 Nippon Kokan Kabushiki Kaisha Method of high speed gas shielded arc welding
US4213027A (en) * 1979-03-06 1980-07-15 Vsesojuzny Nauchno-Issledovatelsky Proektno-Konstruktorsky i Teknologichesky Institut Elektrosvarochnogo Oborudovania Method of treating electrodes intended for operation in argon as cathodes of an electric arc
US5686002A (en) * 1996-08-12 1997-11-11 Tri Tool Inc. Method of welding
US6371359B1 (en) * 1999-04-02 2002-04-16 Nippon Sanso Corporation Stainless steel pipe and joining method thereof
US20020190033A1 (en) * 2001-05-11 2002-12-19 Linde Aktiengesellschaft Tandem welding shielding gases
US6749002B2 (en) * 2002-10-21 2004-06-15 Ford Motor Company Method of spray joining articles
EP2058079A1 (de) * 2006-08-02 2009-05-13 Taiyo Nippon Sanso Corporation Tandem-schutzgasmetalllichtbogenschweissen und bei dem verfahren verwendete(r) schweissbrenner und schweissvorrichtung
CN103302384A (zh) * 2012-03-09 2013-09-18 株式会社神户制钢所 纵列式气体保护电弧焊接方法
CN104028880A (zh) * 2013-03-06 2014-09-10 青岛四方庞巴迪铁路运输设备有限公司 一种铝型材车钩板的焊接工艺方法

Families Citing this family (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JP4786402B2 (ja) * 2006-04-17 2011-10-05 新日本製鐵株式会社 Ｕｏｅ鋼管の製造方法

Citations (8)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2756311A (en) * 1954-03-24 1956-07-24 Union Carbide & Carbon Corp High-speed tandem arc working
US2868954A (en) * 1955-01-10 1959-01-13 Union Carbide Corp Gas shielded multi-arc welding
US3007033A (en) * 1959-04-15 1961-10-31 Union Carbide Corp Inert gas shielded metal arc welding
US3278720A (en) * 1964-02-12 1966-10-11 Reynolds Metals Co Method and apparatus for welding metal members
US3309491A (en) * 1966-06-13 1967-03-14 Charles T Jacobs Consumable anode electrode arc welding
US3596051A (en) * 1970-03-20 1971-07-27 Nippon Koran Kk Method and apparatus for forming t-welds
US3644697A (en) * 1968-04-27 1972-02-22 Alfred Krahl Protective gas for arc welding
US3704358A (en) * 1970-12-14 1972-11-28 Kawasaki Steel Co Submerged-arc both-side butt welding method of a square groove

Family Cites Families (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
NL138853B (nl) * 1967-10-27 1973-05-15 Schelde Nl Werkwijze voor het poederdek- of gasbeschermd booglassen met een afsmeltbare elektrode.

Patent Citations (8)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2756311A (en) * 1954-03-24 1956-07-24 Union Carbide & Carbon Corp High-speed tandem arc working
US2868954A (en) * 1955-01-10 1959-01-13 Union Carbide Corp Gas shielded multi-arc welding
US3007033A (en) * 1959-04-15 1961-10-31 Union Carbide Corp Inert gas shielded metal arc welding
US3278720A (en) * 1964-02-12 1966-10-11 Reynolds Metals Co Method and apparatus for welding metal members
US3309491A (en) * 1966-06-13 1967-03-14 Charles T Jacobs Consumable anode electrode arc welding
US3644697A (en) * 1968-04-27 1972-02-22 Alfred Krahl Protective gas for arc welding
US3596051A (en) * 1970-03-20 1971-07-27 Nippon Koran Kk Method and apparatus for forming t-welds
US3704358A (en) * 1970-12-14 1972-11-28 Kawasaki Steel Co Submerged-arc both-side butt welding method of a square groove

Cited By (14)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4100389A (en) * 1975-12-24 1978-07-11 Nippon Kokan Kabushiki Kaisha Method of high speed gas shielded arc welding
US4213027A (en) * 1979-03-06 1980-07-15 Vsesojuzny Nauchno-Issledovatelsky Proektno-Konstruktorsky i Teknologichesky Institut Elektrosvarochnogo Oborudovania Method of treating electrodes intended for operation in argon as cathodes of an electric arc
US5686002A (en) * 1996-08-12 1997-11-11 Tri Tool Inc. Method of welding
US6371359B1 (en) * 1999-04-02 2002-04-16 Nippon Sanso Corporation Stainless steel pipe and joining method thereof
US6969818B2 (en) * 2001-05-11 2005-11-29 Linde Aktiengesellschaft Tandem welding shielding gases
US20020190033A1 (en) * 2001-05-11 2002-12-19 Linde Aktiengesellschaft Tandem welding shielding gases
US6749002B2 (en) * 2002-10-21 2004-06-15 Ford Motor Company Method of spray joining articles
EP2058079A1 (de) * 2006-08-02 2009-05-13 Taiyo Nippon Sanso Corporation Tandem-schutzgasmetalllichtbogenschweissen und bei dem verfahren verwendete(r) schweissbrenner und schweissvorrichtung
US20090236320A1 (en) * 2006-08-02 2009-09-24 Taiyo Nippon Sanso Corpration Method for tandem gas metal arc welding, and welding torch and welding apparatus used therefor
EP2058079A4 (de) * 2006-08-02 2009-11-04 Taiyo Nippon Sanso Corp Tandem-schutzgasmetalllichtbogenschweissen und bei dem verfahren verwendete(r) schweissbrenner und schweissvorrichtung
US8461471B2 (en) 2006-08-02 2013-06-11 Taiyo Nippon Sanso Corporation Tandem gas metal arc welding
CN103302384A (zh) * 2012-03-09 2013-09-18 株式会社神户制钢所 纵列式气体保护电弧焊接方法
CN103302384B (zh) * 2012-03-09 2015-09-23 株式会社神户制钢所 纵列式气体保护电弧焊接方法
CN104028880A (zh) * 2013-03-06 2014-09-10 青岛四方庞巴迪铁路运输设备有限公司 一种铝型材车钩板的焊接工艺方法

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Publication number Publication date
ES442954A1 (es) 1977-04-16
JPS5236941B2 (de) 1977-09-19
CA1040718A (en) 1978-10-17
DE2552495A1 (de) 1976-06-10
DE2552495C2 (de) 1983-09-08
IT1049705B (it) 1981-02-10
JPS5169436A (de) 1976-06-16

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